Nozzle plate containing multiple micro-orifices for cascade impactor and method for manufacturing the same

ABSTRACT

A nozzle plate containing multiple micro-orifices for the cascade impactor and a method for manufacturing the same are disclosed. The nozzle plate is formed by a series of semiconductor processes, including lithography, etching and electroplating. The nozzle plate comprises a plate body and a plurality of micro-orifices formed on the plate body. The orifice has a diameter which gradually expands in the direction away from the bottom of the plate body to achieve a smooth inner surface, allowing particles to pass therethrough smoothly without being clogged in the nozzle plate.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to aerosol sampling technology, and moreparticularly to a nozzle plate containing multiple micro-orifices foruse in a cascade impactor and a method for manufacturing the same.

2. Description of the Related Art

The Micro-Orifice Uniform Deposit Impactor (MOUDI) invented by MSPCorporation has been widely used for size-classified aerosol sampling.Each stage of the MOUDI consists of a nozzle plate with a plurality ofnozzles and an impaction plates to collect particles of a specific sizerange. By decreasing the nozzle diameter and increasing the air jetspeed in the nozzle from the top to the bottom stages, the MOUDI is ableto collect particles of subsequently smaller size ranges. In a 10 stageMOUDI, the cutoff aerodynamic diameter of the stage 0 to 10 is 18, 10.0,5.6, 3.2, 1.8, 1.0, 0.56, 0.32, 0.18, 0.1, 0.056 μm, respectively, andthere is a final after filter to collect particles smaller than 0.056μm. To classify very small particles, the nozzle plates of the last 4impaction stages, or stage 7 to 10, use 900-2000 micro-orifices with thediameter ranging from 140 to 52 μm to collect particles ranging from0.32 to 0.056 μm in diameter.

U.S. Pat. No. 6,431,014 disclosed an improved MOUDI design with a seriesof differential pressure sensors for measuring the pressure drop acrossthe nozzle plates. Additionally, the influence of particle accumulationand blockage in the micro-orifices on the performance of the MOUDI isalso briefly discussed. The clogged orifices may cause the cut-point ofthe impactor to change which leads to measurement errors. The dustaccumulation problem in the nozzle can be eliminated by periodiccleaning. However, an improper cleaning method, such as high intensityultrasonic cleaning, may damage the nozzle plates whose wall thicknessto define the nozzle diameter is very thin.

Ji et al. (2006) observed the 6^(th) to 8^(th) stage nozzle plate of a8-stage MOUDI by using an electron microscope, and the results werepublished in a journal paper (Ji, J. H., Bae, G. N., Hwang, J., 2006.Observation and evaluation of nozzle clogging in a micro-orificeimpactor used for atmospheric aerosol sampling, Particulate Sci.Technol. 24: 85-96). In the study, nozzle clogging caused by particledeposition in the nozzle was observed. The collection efficiency curveswere shifted to that corresponding to smaller orifice sizes, and the 50%cutoff sizes were much smaller than those specified by the manufacturerfor the three stages with nozzles less than 400 μm in diameter. Thepressure drops across the clogged nozzles were also higher than thenominal values given by the manufacturer.

The inventor of the present invention used an optical microscope toobserve the micro-orifices of the nozzle plate of the last severalstages of the MOUDI. An uneven inner surface of the micro-orifices wasobserved (see FIG. 11). In the current method, the major part of thenozzle is made by the wet etching process while the final bottom part ofthe orifice has to be made by laser drilling to define a known orificediameter. Due to the thickness limitation of laser drilling used tomanufacture the orifice, the wall thickness D1 at the bottom side ofeach micro-orifice is only about 10 μm. This is the main reason whythere exists an abrupt step at the bottom of the orifice which rendersclogging of particles easily. Besides, this fragile structure preventthe nozzle plates from being cleaned effectively, such as by anultrasonic cleaner. Improvement of the structure and the shape of themicro-orifices for the nozzle plate is therefore critically needed.

SUMMARY OF THE INVENTION

It is the main object of the present invention to provide a nozzle platewith multiple micro-orifices for a cascade impactor and a method formanufacturing the same, wherein the micro-orifices of the nozzle platehave a smooth inner surface, avoiding clogging of particles in thenozzle plate.

It is another object of the present invention to provide a nozzle platefor a cascade impactor and a method for manufacturing the same, whereinthe uniform wall thickness and sturdy structure of the micro-orificesfacilitate cleaning by an ultrasonic cleaner.

To achieve these and other objects of the present invention, a nozzleplate for a multi-stage cascade impactor comprises a plate body, and aplurality of micro-orifices formed on the plate body and cutting throughtop and bottom sides of the plate body. Each micro-orifice has a smoothinner surface and a diameter expanding gradually in direction from thebottom side of the plate body toward the top side thereof. Further, thenumber of the micro-orifices is preferably within 50-10000, and thediameter of each micro-orifice at the bottom side of the plate body iswithin 45-410 μm. The nozzle plate further comprises a plurality ofannular protrusions protruded from the bottom side of the plate bodyaround each micro-orifice.

To achieve these and other objects of the present invention, a methodfor making a nozzle plate containing multiple micro-orifices comprisesthe steps of:

(1) depositing a seed layer on a substrate; (2) coating the seed layerwith a layer of first photoresist, radiating UV light through a firstmask onto the first photoresist, and then developing the firstphotoresist; (3) etching the seed layer and removing the firstphotoresist, so as to form a plurality of through holes on the seedlayer that cut through top and bottom sides of the seed layer; (4)coating a sacrificial layer on the substrate and the seed layer; (5)depositing a metal mask film on the sacrificial layer; (6) coating alayer of second photoresist on the metal mask film, radiating UV lightthrough a second mask onto the second photoresist, and then developingthe second photoresist; (7) etching the metal mask film and removing thesecond photoresist, so as to form a plurality of protrusions on thesacrificial layer; (8) etching the sacrificial layer until the substrateand the seed layer are exposed to the outside; (9) electroplating ametal material onto the seed layer; and (10) removing the substrate, theseed layer and the sacrificial layer. Further, the metal material usedduring step (9) is a mix of nickel and cobalt.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic drawing showing the fabrication of a nozzle platecontaining a plurality of micro-orifices for cascade impactor inaccordance with the present invention (I).

FIG. 2 is a schematic drawing showing the fabrication of a nozzle platecontaining a plurality of micro-orifices for cascade impactor inaccordance with the present invention (II).

FIG. 3 is a schematic drawing showing the fabrication of a nozzle platecontaining a plurality of micro-orifices for cascade impactor inaccordance with the present invention (III).

FIG. 4 is a schematic drawing showing the fabrication of a nozzle platecontaining a plurality of micro-orifices for cascade impactor inaccordance with the present invention (IV).

FIG. 5 is a schematic drawing showing the fabrication of a nozzle platecontaining a plurality of micro-orifices for cascade impactor inaccordance with the present invention (V).

FIG. 6 is a schematic drawing showing the fabrication of a nozzle platecontaining a plurality of micro-orifices for cascade impactor inaccordance with the present invention (VI).

FIG. 7 is a schematic drawing showing the fabrication of a nozzle platecontaining a plurality of micro-orifices for cascade impactor inaccordance with the present invention (VII).

FIG. 8 is a schematic drawing showing the fabrication of a nozzle platecontaining a plurality of micro-orifices for cascade impactor inaccordance with the present invention (VIII).

FIG. 9 is a schematic drawing showing the fabrication of a nozzle platecontaining a plurality of micro-orifices for cascade impactor inaccordance with the present invention (IX).

FIG. 10 is a schematic drawing showing the fabrication of a nozzle platecontaining a plurality of micro-orifices for cascade impactor inaccordance with the present invention (X).

FIG. 11 is a schematic sectional view of a nozzle plate for cascadeimpactor made according to the prior art design.

DETAILED DESCRIPTION OF THE INVENTION

Referring to FIGS. 1-10, a method for the fabrication of a nozzle platehaving multiple micro-orifices for the cascade impactor in accordancewith the present invention includes the steps of:

(1) depositing a seed layer 22 on a glass substrate 20, as shown in FIG.1, wherein copper or chromium can be used to deposit the seed layer 22by a sputtering process, an evaporation process or a chemical vapordeposition (CVD) process; the seed layer 22 has a thickness D2 about 3μm;

(2) coating the seed layer 22 with a layer of first photoresist 24,radiating UV light through a first mask 26 onto the first photoresist24, and then developing the first photoresist 24, as shown in FIG. 2,wherein the first mask 26 has a plurality of transparent regions 261 forthe passing of the applied UV light; for the sake of brevity, only onetransparent region 261 is seen in FIG. 2;

(3) etching the seed layer 22 and removing the first photoresist 24, asshown in FIG. 3, so as to form a plurality of through holes 221 on theseed layer 22 that cut through top and bottom sides of the seed layer22;

(4) coating a sacrificial layer 28 on the glass substrate 20 and theseed layer 22, as shown in FIG. 4, wherein the sacrificial layer 28 canbe prepared from, for example, but not limited to, polyimide (PI);

(5) using copper or chromium to deposit a metal mask film 30 on thesacrificial layer 28 by a sputtering process, an evaporation process ora chemical vapor deposition process, as shown in FIG. 5;

(6) coating a layer of second photoresist 32 on the metal mask film 30,radiating UV light through a second mask 34 onto the second photoresist32, and then developing the second photoresist 32, as shown in FIG. 6,wherein the second mask 34 has a plurality of circular opaque regions341 at locations corresponding to the first through holes 221 on theseed layer 22;

(7) etching the metal mask film 30 and removing the second photoresist32, as shown in FIG. 7, so as to form a plurality of protrusions 301 onthe sacrificial layer 28; for the sake of brevity, only one circularopaque region 341 and one protrusion 301 are respectively seen in FIGS.6 and 7;

(8) etching the sacrificial layer 28 until the glass substrate 20 andthe seed layer 22 are exposed to the outside, as shown in FIG. 8;

(9) electroplating a metal material 36 onto the seed layer 22 to adesired thickness D3, as shown in FIG. 9, wherein the metal material canbe, but not limited to, a mix of nickel and cobalt, and the thickness D3of the metal material 36 is 150 μm; and

(10) removing the substrate 20, the seed layer 22 and the sacrificiallayer 28, thereby obtaining a nozzle plate 10, as shown in FIG. 10,which is to be processed further through a series of cutting andhole-drilling processes for installation in a multi-stage cascadeimpactor.

Referring to FIG. 10, a nozzle plate 10 for cascade impactor inaccordance with the present invention is made through a series ofsemiconductor processes, including lithography, etching andelectroplating. The nozzle plate 10 comprises a plate body 12 and aplurality of micro-orifices 14 cut through top and bottom sides of theplate body 12. Because the nozzle plate 10 is formed by means ofelectroplating, the micro-orifices 14 have a smooth inner surface and adiameter which expands gradually from the bottom side of the plate body12 toward the top side thereof. Further, the nozzle plate 10 has anannular protrusion 16 protruded from the bottom side around each of themicro-orifices 14.

Further, the smooth inner surfaces of the micro-orifices 14 allowparticles to pass therethrough smoothly without clogging themicro-orifices. Further, the uniform wall thickness and sturdy structureof the micro-orifices 14 facilitate cleaning by an ultrasonic cleanerand improve the convenience of use and the sampling quality. Further,subject to different desired cut-off aerodynamic diameters, the numberof the micro-orifices 14 of the nozzle plate 10 and their final orificediameter can be 900/140 μm, 900/90 μm, 2000/55 μm, 2000/52 μm, 980/49μm, 1650/450 μm or 2000/55 μm. Preferably, the number of themicro-orifices 14 is within 50-10000, and the diameter is within 45-410μm.

Although particular embodiments of the invention have been described indetail for purposes of illustration, various modifications andenhancements may be made without departing from the spirit and scope ofthe invention. Accordingly, the invention is not to be limited except asby the appended claims.

1. A nozzle plate for a multi-stage cascade impactor, comprising: aplate body; and a plurality of micro-orifices formed on the said platebody and cutting through top and bottom sides of the said plate body,each said micro-orifice having a smooth inner surface and a diameterexpanding gradually in direction from the bottom side of said the platebody toward the top side thereof.
 2. The nozzle plate as claimed inclaim 1, wherein the number of the said micro-orifices is within50-10000.
 3. The nozzle plate as claimed in claim 1, wherein the numberof the said micro-orifices is within 900-2000.
 4. The nozzle plate asclaimed in claim 1, further comprising a plurality of annularprotrusions protruded from the bottom side of said plate body aroundeach said micro-orifice.
 5. The nozzle plate as claimed in claim 1,wherein the diameter of each said micro-orifice at the bottom side ofsaid plate body is within 45-410 μm.
 6. A method for making a nozzleplate containing multiple micro-orifices, comprising the steps of: (1)depositing a seed layer on a substrate; (2) coating the said seed layerwith a layer of first photoresist, radiating UV light through a firstmask onto said first photoresist, and then developing the firstphotoresist; (3) etching said seed layer and removing the said firstphotoresist, so as to form a plurality of through holes on said seedlayer that cut through top and bottom sides of said seed layer; (4)coating a sacrificial layer on said substrate and said seed layer; (5)depositing a metal mask film on said sacrificial layer; (6) coating alayer of second photoresist on the said metal mask film, radiating UVlight through a second mask onto the said second photoresist, and thendeveloping the second photoresist; (7) etching the said metal mask filmand removing the said second photoresist, so as to form a pluralityprotrusions on the said sacrificial layer; (8) etching the saidsacrificial layer until said substrate and the said seed layer areexposed to the outside; (9) electroplating a metal material onto thesaid seed layer; and (10) removing the said substrate, the said seedlayer and the said sacrificial layer.
 7. The method for making a nozzleplate containing multiple micro-orifices as claimed in claim 6, whereinthe metal material used during step (9) is a mix of nickel and cobalt.